A computer from 1948 in ICMP packets

ICMP delay-line memory (or, in this case, ICMP Williams tube). That it's used for maintain the state of a simulation of the Manchester Baby is too cool!

See also:

Pingfs – a filesystem where the data is stored only ICMP Echo packets: https://news.ycombinator.com/item?id=9844725

Sorting 1 million 8-decimal-digit numbers with 1 MB of RAM - https://stackoverflow.com/a/13000176